When an AC induction motor with a squirrel-cage rotor starts up, power is applied to the stator windings. The power sets up an electric field that rotates about the stator, generates a rotating magnetic field about the rotor, and urges the rotor into motion. When the motor is running at full speed, the speeds of the stator and rotor fields differ by a small amount called slip. The slip is usually a few percent of the stator frequency, and this difference is the rotor frequency. At start-up, however, the rotor is at rest, and the difference is the full amount of the stator frequency. Thus at start-up, stator and rotor currents can reach five or six times their normal operating values. The power applied to the stator circulates in the stator windings, while resulting rotor currents circulate in rotor bars and end rings (a "squirrel-cage") placed into the rotor. FIG. 1 shows the general arrangement of an AC induction motor, while FIG. 2 depicts a "squirrel-cage" rotor.
The very high currents at start up can damage or destroy starting equipment, and constitute a great burden on a plant's electrical equipment and ultimately on electric utilities. A variety of methods have been used to reduce starting currents. The problem can be solved by means of external electrical controls. A motor can be started at reduced voltage, or a "soft-start" control algorithm can be employed. These methods are not suitable for starting up loads requiring high start-up torque. A solution that adapts the motor hardware itself would be self-contained, reliable, and would not involve extra external circuits or controls.
Motors may take advantage of the skin effect of electricity by increasing rotor resistance during startup or reducing rotor resistance after startup. The skin effect teaches that electricity of high frequency flows only in an outer skin of a conductor, while electricity of low frequency can penetrate conductors and more fully utilize the conductors available. At start-up, rotor frequency is high, and the skin effect limits the penetration of rotor currents into the rotor bars and end rings. Once the motor is started, however, rotor frequency is low, and the full depth of a conductor is available to rotor currents. Previous attempts to use the skin effect to reduce start-up current and increase locked rotor torque have mainly involved the details of the rotor slots. Other non-electromagnetic methods for lowering startup current and increasing locked rotor torque are mechanically involved and add parts and complexity to the motor.
Engineers have tried varied ways to solve the problem of high start-up current by increasing the resistance on start up or by lowering resistance during running. Thus, Higashi, U.S. Pat. No. 4,885,494, proposes starting up an induction motor, and then filling spaces in the rotor with liquid nitrogen to achieve superconductivity once the motor is running. While this method achieves the desired result, it is not practical. Other methods with mechanical complexity are shown in Plumer, U.S. Pat. No. 4,720,647, Lundquist, U.S. Pat. No. 5,068,560, and Gupta, U.S. Pat. No. 5,751,082. These methods depend on the increased speed and rotational inertia of the motor after startup to force greater masses of conductive material into contact with the end rings, through springs or flyweights. These methods have the double disadvantage of mechanical complexity and decreased performance when the motor becomes old, hot and dirty.
Tapered end rings have been used for several reasons, as in Lundquist, U.S. Pat. No. 5,068,560, Hibino, U.S. Pat. No. 5,182,483, and Shafer, U.S. Pat. No. 5,185,918. In Lundquist, end rings of tapered cross section are used for mating parts to one another, and possibly for ease of manufacture. Hibino casts rotor bars into the rotor stack, and likely uses a slight draft angle on the end rings in order to remove the cast rotor from the casting tool. Shafer uses tapered rotor bars, and subsequently squares them up by plasma spraying end rings onto the rotor bars. None of these methods use a designed, controlled taper to significantly reduce rotor currents and increase locked rotor torque on start up.
Various types of rotor slot designs attempt to use the flux leaking across the rotor slots as a device to increase rotor resistance during startup. The phenomenon is documented in places such as the Louis Allis Pacemaker.RTM. motor catalog, in their explanation of NEMA motor design types. Such a design crowds the rotor current toward the outside diameter of the rotor during startup. As the rotor speed increases the current begins to penetrate deeper into the bar. This electromagnetic effect occurs in "deep bar" type rotors and "double cage" type rotors.
Double cage rotors attempt to increase resistance on startup, as shown in Marks Standard Handbook for Mechanical engineers, 7.sup.th ed. (1967) at 15-63, and Neumann, U.S. Pat. No. 4,831,301. These motors use an outer squirrel cage of higher resistance material, such as brass, and an inner squirrel cage of low resistance material, such as copper. Upon startup, the skin effect limits rotor currents by means of the high resistance brass material in the outer cage, while afterwards, the skin effect is lessened, and the rotor current can penetrate the rotor to take advantage of the copper cage, lowering resistance during normal operation. A "double cage" rotor may be considered as merely a very complex shaped "deep bar" type rotor. The disadvantage of this construction is the extra labor, expense and inconvenience of two different types of rotor bar material. What is needed is a construction that reduces rotor current on start up without added parts or complexity.